Aligned cholesteric liquid crystal inks

ABSTRACT

In color printing, and in the fine arts, cholesteric liquid crystal (CLC) color inks are known to possess much higher color saturation and brightness than conventional pigment and dyed based inks. However, prior art CLC ink formulations are inconvenient because in the liquid phase they have to be confined in cells, and in the solid phase, they have to be applied at high temperature, and have to be aligned by some means to produce the optimum color. This invention solves the problem encountered in the CLC prior art, by making pre-aligned CLC platelets or flakes of appropriate thickness and size and mixing them in appropriate host fluids producing a novel CLC ink which can be applied at room temperature and without the need for alignment. The new pre-aligned room temperature CLC ink can be used as a substitute for conventional inks in almost all printing and plotting, and manual drawing and painting. Using the notch filter CLC platelets, the brightness is further enhanced. This invention teaches the CLC ink concepts, its applications and method of manufacturing.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to the field of color inks and paints usedin the printing, signage, fine and decorative arts industries.

[0003] 2. Description of Related Art

[0004] David Makow in Color, Vol.11, No. 3, p.205 (1986) has shown thatcholesteric liquid crystals (CLC), and in particular, the CLC polymers(U.S. Pat. No. 4,410,570), possess color properties and effects that arenot possible to obtain by conventional dyes and pigments, including:additive color properties; higher saturation and wider color gamut.However, in their present forms, liquid crystal coatings cannot be usedas general purpose color inks and paints for the printing, signage, fineand decorative arts industries. CLC's in the liquid phase are notpossible to use unless they are somehow encapsulated. The CLC polymercoatings, on the other hand, are solid at room temperature, and as Makowshowed, produce remarkable color effects and are highly stable. TheseCLC polymers are still inconvenient for general purpose applicationsbecause they have to be applied at high temperatures. Thepolysiloxane-base CLC polymers are applied at 140° C. in the liquidcrystal phase and its molecules must be aligned to form the helicalconfiguration with the helix axis perpendicular to the substrate (paperor canvas). This constrains the use of CLC polymers only in specialapplications and only by specialists.

[0005] This invention shows that by making CLC polymers into flat flakesor platelets having the helical axis normal to the platelets surface andmixing them in a suitable fluid, the prior art problems are solved,making it possible for CLC polymers to be conveniently used for generalpurpose applications exploiting their remarkable color properties. Thisis a CLC ink which is applied at room temperature, and no furtheralignment by the user is needed, since the platelets are already in theproper helical configuration.

SUMMARY OF THE INVENTION

[0006] The principal object of the present invention is to provide amethod for producing CLC flat flakes or platelets.

[0007] Another object of this invention is to make novel CLC color inkswhich can be applied at room temperature and after drying, retain theirremarkable color effects.

[0008] Another object of this invention is to provide a method formaking CLC color inks using notch filter platelets which result in 100%reflection of ambient light producing the brightest and most saturatedcolors.

[0009] Another object of this invention is to provide low costpolarizers and polarizing filters.

[0010] Another object of this invention is to to provide a broadbandcircular polarizer based on CLC materials.

[0011] Another object of this invention is to provide a new method formaking micro-polarizer arrays needed for 3-D stereo displays.

[0012] Yet another object of this invention is to provide novel colorCLC pens, pencils, and crayons for painting and printing applications:

[0013] These and other objects will become apparent when the preferredembodiments are described.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is an illustration of cholesteric liquid crystal polymerplatelets which are used in the novel ink.

[0015]FIG. 2 illustrates cross-sections of individual platelets of thesimple kind and the notch filter kind.

[0016]FIGS. 3a-c illustrate three methods for manufacturing CLCplatelets and inks.

[0017] FIG.4 illustrates methods of laminating CLC layers and retarderlayer for producing notch filter CLC platelets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] cholesteric Liquid Crystal Inks

[0019] The present invention depends on the well known properties ofchiral liquid crystals, CLC, ( also called cholesteric liquid crystals)described in the following references: S. D. Jacobs et. al., Journal ofthe Optical Society of America, B, Vol. 5(9), pp 1962-1978 (September1988); ii)-Martin Schadt and Jurg Funfschilling, Society of InformationDisplays, SID 90 DIGEST, p 324 (1990); and iii)—Robert Maurer, et. al.,Society of Information Displays, SID 90 DIGEST, p 110 (1990). Theseliquid crystals spontaneously order themselves in an optically activestructure of a left handed (LH) helix or a right handed (RH) helix witha helix pitch P, and an optical axis which coincides with the helixaxis. FIG. 1a shows an RH, CLC film 1 (cross section) prepared with itsoptical axis 2 perpendicular to the film. It exhibits the property ofselective reflection when a monochromatic beam 3 of wavelenght λ,propagating along the helix axis satisfies the relationship

λ=λ₀=n_(a)P,  (1)

[0020] where n_(a) is the average refractive index of the CLC materialand P is its pitch. Unpolarized light 3 with wavelength λ=λ₀ incident onthe film will interact with the helix structure and causes thereflection of 50% of its intensity as right circularly polarized light 3a (RCP), and the other 50% is transmitted as left circularly polarizedlight 3 b (LCP). On the other hand, if the incident light 4 has one ormore wavelengths that are not equal to So, all the light is transmitted.We remark that equation (1) is strictly valid in the case where theangle of incidence θ (measured from the helix axis) is zero. For anon-zero value of θ, the effective value of λ_(O) shifts to a shorterwavelength λ_(θ given by)

λ_(θ)=λ_(o)[cos {sin−1(sinθ/n_(a))}]  (1a)

[0021] In all subsequent discussions in this application, whenever θ≠0it is implied that λ_(o means λ) _(θ)as given by Eq. (1 a). If the filmhad an LH helix, FIG. 1b, and the incident unpolarized light 6 satisfiesλ=λ_(o), 50% of the selectively reflected polarized light 6 a would havethe LCP state, and the other 50% transmitted part 6 b would have the RCPstate. The selective reflection wavelengths according to Eq. 1 is tunedby tuning the pitch length which is a material property that is variedby varying the chiral concentration or the concentration of themesogenic side-groups (U.S. Pat. No. 4,410,570). Thus the CLC materialsare prepared to produce the three additive primary colors; red, green,and blue. It is important to note that this selective reflectionpolarizing property does not involve or depend on an absorptivemechanism as in the case of conventional color pigments, dyes and sheetpolarizers.

[0022] A fundamental property of light is that it can have only twoindependent, mutually orthogonal polarization states, either circular,LCP and RCP states, or linear states. Other polarization properties oflight used in this invention are shown in FIGS. 1c-1 e. FIG. 1c showsthat an LCP light 8 incident on a metallic reflector 9 is converted intoan RCP light 10 because the metal causes a phase shift of 180° C.between the independent electric field vector components. A quarter-waveretarder 11, FIG. 1d, causes a 90° C. phase shift and converts acircular light 12 into linear 13, and a linear light 14 into circular15. In FIG. 4e, a half-wave retarder 16 converts RCP light 17 into LCPlight 18 and vice versa by causing a phase shift between the independentelectric field vector components

[0023] The present invention relies on CLC materials in the solid stateat the operating temperature. Such CLC polymers have been synthesized inthe LH and RH formulations (See M. L. Tsai et al, Appl. Phys. Lett., 54,2395 (1989)). These polymers are brittle. I have exploited thisbrittleness property in an experiment to prove that I can make smallflakes or platelets which when applied (easily transferred) to adifferent substrate, retained their selective reflection property, i.e,the platelets remained. aligned in the helical configuration with thehelix axis normal to the platelet surface. FIG. 2a illustrates typicalCLC flakes or platelets shapes 8. They can have regular or irregulargeometrical shapes, with the average lateral dimension typically morethan 3 times the thickness. Platelets 8 could have average lateraldimensions are in the 4 to 100 microns range (8 to 200 helix pitches),and average thicknesses of 4 to 20 helix pitches. FIGS. 2b and 2 c showsimple CLC platelets 20 a, 20 b which have either LH CLC 20 a or RH CLC20 b helices. These simple platelets while they yield acceptablebrightness and color saturation for many printing applications, theystill waste 50% of the selected color energy. The notch filter plateletsshown in FIGS. 2d and 2 e are preferred because they reflect 100% of thelight, thereby increasing the brightness by a factor of 2. This can beunderstood by referring to the CLC and polarization of light propertiesdescribed above and in FIG.1. In FIG.2d, a platelet 21 for a particularcolor band (e.g. red) consists of two CLC layers, an LH layer 21 a andan RH layer 21 b. A red beam incident on platelet 21 is totallyreflected. 50% of the light is reflected by the LH layer 21 a as an LCPlight, and the remaining 50% is transmitted through the LR layer 21 a asan RCP beam. Said transmitted RCP beam is subsequently reflected by theRH layer 21 b and is then transmited again through layer 21 a to theobserver. Thus, all the incident light is reflected. The same result isachieved if the RH layer 21 b is replaced with a half-wave retarderlayer 21 c and a second LH layer 21 d as shown in FIG.2e. In this case,the RCP light transmitted through layer 21 a is converted to LCP lightby the retarder 21 c which in turn is reflected by the second LH layer21 d. The reflected LCP is transmitted again (in the reverse direction)through retarder 21 d and is converted back to RCP light that istransmitted again ( in the reverse direction) through the first LH layer21 a, completing the 100% reflection of the incident red beam. The samehappens for the other colors by means of appropriately tuned platelets.These platelets of the simple 20 and notch filter 21 types are mixed ina suitable fluid producing a CLC ink which is then used in printing,drawing, painting and other imaging applications. These CLC inks areapplied at room temperature and do not need alignment by the user,solving prior art problems encountered in the Makow Reference.Conventional pigments and dye inks filter colors by an absorptionmechanism and are applied to white background, such as paper substrates.The CLC inks, on the other hand, are reflective (see propertiesdescribed above, FIG. 1) and are applied to black background such asblack paper. The CLC inks are applied to the black substrate such thatthe platelets lie parallel to the substrate surface, and the CLC helicalaxes are normal to said substrate surface. Exploiting the remarkableadditive and color saturation properties, red, green and blue CLC inksare sufficient to generate all colors. These CLC color inks are mixedbefore application to the substrate or they are mixed sequentially asthey are applied in turn onto the substrate. To my knowledge, no priorart has taught how to produce CLC inks, applied at room temperature (oroperating temperature), that reflect 100% of the incident color, andwithout the need for alignment.

[0024] The CLC ink according to this invention comprises the pre-alignedCLC flakes or platelets and a suitable fluid. Said fluid is well knownin the ink art ( see Chapter 18, p 523 in J. Michael Adams, PrintingTechnology, 3 rd Ed., Delmar Publishers, Inc., Albany, N.Y., 1988) andis selected depending on the applications. It further comprises vehiclesand additives chosen for tackiness, drying speed, adhesion tosubstrates, printing or painting methods, and other properties.

[0025] Manufacturing Method

[0026]FIGS. 3a-c describe methods and apparatuses used for highthroughput economical manufacturing of CLC platelets. Apparatus 22 inFIG.3a comprises a first belt 32 rotated continuously by means ofrotating drums 24, 25, and a second belt 34 rotated by drums 36, 37 inthe opposite direction of first belt 32. The first belt 32 carries thealigned coating of a CLC, while the second belt 34 is allowed to pressagainst the first belt in order to remove the CLC coating by adhesivemeans. This process of coating and removal of aligned CLC layers and theproduction of the final product, the platelets or flakes are carried outcontinuously according to the following steps:

[0027] 1. The starting CLC polymer material in a molten state in acontainer 26 is coated onto belt 23 by means of a roller 27 othercoating means such as spraying and casting are possible).

[0028] 2. While the coated belt is in motion, a knife edge means 28 isused to smooth the CLC film, maintains a uniform and repeatablethickness, and aligns the CLC molecules such that the helix axis isperpendicular to the belt surface. The alignment step is a crucialelement for practicing this invention. The excess CLC material 29 isrecycled.

[0029] 3. The pre-aligned CLC film then passes through an auxiliaryalignment means 30 (if necessary) which applies electric or magneticfields in the proper orientation to ensure that the entire film isaligned in the helical form.

[0030] 4. Above steps are carried out above the glass temperature andbelow the clearing temperature of the CLC polymer. Forpolysiloxane-based CLC polymers, this coating and aligning temperature(processing temperature)is between 120° C. and 150 ° C. Other CLCpolymers may require different processing temperatures.

[0031] 5. The aligned CLC film then passes through a drying and coolingchamber 31 and the desired pre-aligned CLC film 32 below the glasstemperature is brittle and can be transferred adhesively by the secondbelt 34.

[0032] 6. The second belt 34, rotating in the opposite direction offirst belt, is coated by means of a roller 38 (spraying or otherwell-known means may be used) with an adhesive. Said adhesive passesthrough chamber 39 for drying and maintaining an optimum operatingtemperature, and other adhesive properties. The adhesive could be watersoluble polyvinyl alcohol or other adhesives which A can be dissolved insuitable low cost solvents that have minimum environmental impact. Someadhesive may be chosen to be brittle when dry.

[0033] 7. The optimized adhesive coating e is pressed by means of drum37 onto CLC film a on drum 25. This action transfers the CLC film frombelt 23 to belt 34. Drums 25 and 37 have a rubber surface that ensuresoptimum transfer of CLC to the adhesive. My experiments indicated thatthe brittleness of polysiloxane CLC polymer transfers in the form ofsmall platelets or flakes.

[0034] 8. The transferred CLC on the adhesive is passed through a cooler37 a which cools the combined coating to low enough temperature toensure the brittleness of both CLC coating and the adhesive coating.While polysiloxane based CLC polymer is naturally brittle at roomtemperature, other CLC polymers may not be brittle enough for thesubsequent step. By cooling to cryogenic temperature such as that ofliquid carbon dioxide or liquid nitrogen, it is well known that polymers(CLC's and adhesives become brittle.

[0035] 9. The brittle CLC and adhesive are removed by means of anultrasonic air jet 41 or an air jet mixed with fine powder abrasive. TheCLC on adhesive that is not removed by the ultrasonic means is scrubbedoff by means of a scrubber 42. The flakes of CLC on adhesive arecollected in a container 43 and are poured into container 44.

[0036] 10. The CLC on adhesive mixture is further broken into thedesired average flake or platelet size. The adhesive is subsequentlydissolved off and separated from the CLC flakes which are dried andmixed with the appropriate fluid to produce CLC ink.

[0037] 11 The process steps 1-10 for producing aligned CLC flakes arerepeated continuously as belts 23 and 34 continue to counter rotate.

[0038]FIG. 3b shows another embodiment 45 for producing aligned CLCflakes that uses only a single belt. The embrittled aligned CLC filmpasses through an ultrasonic bath 46 which imparts intense ultrasonicenergy to the CLC film causing it to flake-off.

[0039] Yet another embodiment 47 for producing aligned CLC platelets andsimultaneously produce the final CLC ink (with minimum steps) is shownin FIG. 3c, comprising: a belt 23; two drums 24, 25; a means 48 forcoating, and aligning CLC films; and a means for transferring saidfilms. The transfer means further comprises one or more transfer belts49, 49 a, 49 b, coated respectively with adhesives by means of rollers50, 50 a, 50 b. The rollers 50, 50 a, 50 b coat each of their respectivebelts with a random adhesive pattern. These patterns are designed totransfer CLC flakes with a predetermined average size. The belts 49, 49a, 49 b are immersed in solvent container 51 which dissolves off theadhesive and precipitates the flakes with a predetermined average sizethat are ready for use in inks. In this case the solvent may be theappropriate fluid needed for the final CLC ink product.

[0040] The coating and alignment means 27, 28, 30, 31, 48 used above forthe simple aligned CLC flake 20 in FIGS. 2b, 2 c can also be used toproduce the notch filter flakes 21 in FIGS. 2d, 2 e by placing in theproper sequence a plurality said coating, aligning, and drying means.Alternatively, FIG.4a shows an embodiment which laminates pre-aligned LHCLC film 53 on a substrate (dispensed from a roll 53 a) with apre-aligned RH CLC film (dispensed from a roll 54 a) using the counterrotating laminating rollers 55, 56 and the final notch filter laminate57 is taken up by roller 57 a. The LH and RH laminate 57 is then brokeninto proper sized flakes for use in CLC ink product. In FIG. 4b, anothernotch filter laminate 63 is produced from laminating pre-aligned LH CLCfilms 58, 60 with a half-wave retarder film 59, said retarder film beinginterposed between said CLC films.

[0041] Many skilled in the art will be able to find other variations ofproducing aligned CLC inks without departing significantly from thebasic teachings of this invention. For instance, if the pre-aligned CLCfilm is not brittle, it is still possible to use it for producingplatelets by well known patterning and etching means. In this casephoto-resist or etch resist patterns are generated which serve toprotect the desired platelets regions, and the exposed regions areetched away by a suitable wet or dry etching means. This would producethe desired platelet size and shape.

[0042] Applications of CLC Inks

[0043] The aligned CLC inks produced based on the teachings of thisinvention can be used in the printing, signage, fine and decorative artsindustries. Unlike prior art, these inks can be dispensed by well knownmeans at room temperature and without the need for further alignment ofthe CLC molecules into the desired helical form. In the CLC ink, thealigned CLC flakes are suspended in a host fluid or a host matrix dependon the printing or imaging application. In a crayon or a pencil form,the host matrix could be a wax or an equivalent sticky material that issolid state at room temperature. This is used by the painter by rubbingoff onto a black paper, the CLC flakes of the appropriate color and thehost matrix. The host fluid could be dispensed from a pen for drawing,paining, plotting, and writing. The ink could be applied by means of abrush, roller, or spray gun. The ink could also be formulated for use inoff-set printing wherein the host fluid is made hydrophobic, or ingravure and flexographic printing wherein the host fluid is formulatedfor printing on plastic substrates, or other substrates. The CLC ink mayalso be used as a toner in electrographic copier and printers (based onxerography process), thermal color printers as well as inkjet printers.According to this invention, color images are produced which featurecolors more saturated and brightness high than can be produced byconventional-pigment and dye based inks. The new method for producingreflective color images generally comprises: aligned CLC color inkshaving at least the three additive primary colors red, green and blue;an ink dispensing tool which applies the CLC ink at ambient temperature;an image source which drives the ink dispensing tool; and a blacksubstrate (paper, canvas, plastic sheet). Color images of thetransmission kind can be produced by applying the CLC color inks to atransparent substrate such as glass, polycarbonate sheets, acrylicsheets, and other plastics. In both the reflective and transmissiveimages, the notch filter CLC 21 produce the brightest and highestsaturation images.

[0044] Aligned CLC inks can be used in other applications such as theproduction of

[0045] 1. Polarizing color filters and filter arrays for displays andother imaging applications, by simply printing the appropriate patternwith OLC inks.

[0046] 2. Broad band polarizers and micropolarizer arrays can also beprinted for use in 3-D stereo imaging, 3-D displays, 3-D printing, and3-D cameras.

[0047] 3. Variable transmission windows.

What is claimed is:
 1. A pre-aligned cholesteric liquid crystal (CLC)color ink which is applied at ambient temperature for producing colorimages having color saturation higher and brighter colors thanconventional absorptive pigment-based or dye-based inks comprising:pre-aligned CLC solid state platelets or flakes wherein the CLCmolecules maintain a helical structure with a helix axis perpendicularto the surface of said platelets, a helix pitch pretuned to give theplatelet the selective reflection property at a desired additive primarycolor (wavelength) band; and a host material in which said pre-alignedCLC platelets are suspended and the combination is applied to asubstrate on which a color image is formed, said host material isformulated for its suitability to a particular image forming means andtype of susbtrate on which the image is formed.
 2. A pre-alignedcholesteric liquid crystal (CLC) color ink system which is applied atambient temperature for producing color images according to claim 1,wherein the ink system further comprises an ink dispensing meanssuitable for tie type of substrate chosen and image forming means.
 3. Apre-aligned cholesteric liquid crystal (CLC) color ink system which isapplied at ambient temperature for producing color images according toclaim 1, wherein the CLC platelet is made of polymer having a glasstemperature considerably higher than the operating temperature.
 4. Apre-aligned cholesteric liquid crystal (CLC) color ink system which isapplied at ambient temperature for producing color images according toclaim 1, wherein the CLC platelet is made of polymer having a glasstemperature considerably higher than ambient temperature or roomtemperature.
 5. A pre-aligned cholesteric liquid crystal (CLC) color inksystem which is applied at ambient temperature for producing colorimages according to claim 1, wherein the CLC platelet is made of apolymer using polysiloxane backbone to which a mesogen sidegroup isattached through a flexible spacer.
 6. A pre-aligned cholesteric liquidcrystal (CC) color ink system which is applied at ambient temperaturefor producing color images according to claim 1, wherein the CLCplatelet is made of a polymer capable of producing left handed helix orright handed helix configurations, and further the helix pitch can betuned by means of chiral additive or mesogenic sidegroup concentrations.7. pre-aligned cholesteric liquid crystal (CC) color ink system which isapplied at ambient temperature for producing color images according toclaim 1, wherein the shape of the platelet is either irregular orregular such as circles, ellipses, rectangle, or polygons.
 8. Apre-aligned cholesteric liquid crystal (CC) color ink system which isapplied at ambient temperature for producing color images according toclaim 1, wherein the average size of the platelet is in the 4-100microns range ( 8 to 200 helix pitches), and. the average thickness is 4to 20 helix pitches, and the average size to thickness ratio is largerthan
 3. 9. A pre-aligned cholesteric liquid crystal (CC) color inksystem which is applied at ambient temperature for producing colorimages according to claim 1, wherein the ink is applied on either aopaque, transparent, black, white or colored substrates.
 10. Apre-aligned cholesteric liquid crystal (CC) color ink system which isapplied at ambient temperature for producing color images according toclaim 1, wherein the ink is applied to substrates made of rigid orflexible materials such as paper, plastics, wood, glass, metal, glass,cloth, or leather.
 11. A pre-aligned cholesteric liquid crystal (CC)color ink system which is applied at ambient temperature for producingcolor images according to claim 1, wherein the platelets are either lefthanded or right handed helices.
 12. A pre-aligned cholesteric liquidcrystal (CC) color ink system which is applied at ambient temperaturefor producing color images according to claim 1, wherein the plateletsare based on the notch filter configuration (21) comprising a laminateof left handed and right handed layers.
 13. A pre-aligned cholestericliquid crystal (CC) color ink system which is applied at ambienttemperature for producing color images according to claim 1, wherein theplatelets are based on the notch filter configuration (21) comprising alaminate of a half-wave retarder interposed between a first and secondleft handed layers.
 14. A pre-aligned cholesteric liquid crystal (CC)color ink system which is applied at ambient temperature for producingcolor images according to claim 1, wherein the host fluid is selectedfor use with printing presses such as those based on off-set,flexographic, gravure and screen printing technologies.
 15. Apre-aligned cholesteric liquid crystal (CC) color ink system which isapplied at ambient temperature for producing color images according toclaim 1, wherein the host material is a clear soft solid matrix made oforganic or inorganic materials such that when pressed against asubstrate, part of the material is transferred to and sticks to saidsubstrate.
 16. A pre-aligned cholesteric liquid crystal (CLC) color inkwhich is applied at ambient temperature for producing color imagesaccording to claim 15, wherein the solid matrix is clear wax.
 17. Apre-aligned cholesteric liquid crystal (CLC) color ink which is appliedat ambient temperature for producing color images according to claim 1,wherein the host material is a water solution of polyvinyl alcohol whichhas an adhesive property mixed with a fast drying agent such as ethyl ormethyl alcohol.
 18. Pre-aligned cholesteric liquid crystal (CLC) solidstate platelets or flakes wherein the CLC molecules are aligned by,alignment means to maintain a helical structure with a helix axisperpendicular to the surface of said platelets, said helix has a pitchwhich is pre-tuned to give the platelet the selective reflectionproperty at a desired additive primary color (wavelength) band, saidplatelets have average size to average thickness ratio in the range of 3to 20.